Polymer films are commonly utilized to produce containers, such as pouches, that are used to contain, transport, and preserve a variety of substances, including but not limited to, foods. These containers are commonly created using a heat sealing process, in which high pressure and temperature is applied to opposing polymer films or laminates to join them together. Through this sealing process, the shelf life of the packaged material is extended through the prevention of contamination of the substance from such processes as microbial contamination or an influx of gases such as oxygen (which can cause rancidity of some packaged food products via oxidation of fats and oils) or water vapor (which can cause staleness of some packaged food products via moisture retention or loss). The temperature at which this seal is created can be relatively high depending on the melting or softening point of the polymer material used as a sealant. The seal initiation temperature (SIT) of the sealant material can be important to commercial packagers since it will influence the operating conditions (e.g. temperature set points) of the packaging machine as well as packaging speeds due to thermal transfer/residence time for the heat from the sealer jaws to soften and fuse the package's sealant material. In general, a lower SIT is desirable as it can allow lower temperature settings for the heated jaw sealers of the packaging machine. By reducing this seal initiation temperature, the process can be made to be faster, more economical, use less electricity, and be more efficient. In addition, lower sealing temperatures may reduce the risk of thermal deformation or distortion of the packaging material, resulting in more attractive pouch appearance on the store shelf.
A typical packaging pouch is a laminate of several films, typically constructed of: A film that can be printed for the marketing of the food product; a barrier film to inhibit the diffusion of oxygen and moisture and thus prolong the shelf-life of the product; and a sealant film which provides hermetic seals that also help prevent ingress of gases or microbes that could shorten the shelf-life of the product or cause spoilage. In some cases, the barrier film and the sealant film can be combined into a single film that provides both functions of gas barrier as well as sealability. Typically, this sealant film is a non-oriented, cast polypropylene or polyethylene-based film. The polyethylene film can also be made via blown film processes well-known in the art.
However, the high seal strengths required for some pouch packaging also make it difficult for the consumer to open the pouch by hand, especially if the retort package is made of all-polymeric films. Scissors or sharp implements must typically be used to open such pouches. To make the pouches more user-friendly, notches can be used to enable the consumer to easily initiate a tear and thus open the pouch. However, such a tear can easily result in “zippering” of the pouch whereby the tear is not uniformly parallel to the top edge of the pouch but can become vertical or diagonal to the top of the pouch and cause a potential loss or spillage of the contents during opening. To rectify this, some solutions involve perforating a tear-line with the notch in order to keep the tear directionally parallel to the top of the pouch and thus prevent zippering. These perforations are often accomplished using mechanical perforators or lasers. Some concerns using perforation techniques are not only additional cost, but also the potential compromising of barrier properties since these techniques are essentially making physical holes in the pouch laminate.
Another method to impart directional tear properties could be to orient the cast polypropylene film typically used in pouch applications. However, the process of orienting such a film—either uniaxially or biaxially—typically diminishes the seal properties in that the seal initiation temperature (SIT) of the film is raised and the overall seal strengths are weaker. Without being bound by any theory, this is believed to be due to the fact that the orientation process aligns the amorphous regions into a more ordered configuration, raising the Tg of the film, and thus, resulting in poorer seal properties. This is why unoriented cast polypropylene works well as a sealant film versus, for example, biaxially oriented polypropylene film (BOPP) which generally functions poorly as a sealant film. (This is assuming that no coextruded highly amorphous/low crystallinity random copolymer heat sealable resins are used as part of the BOPP film.) There is typically a minimum and maximum range for uni-axial orientation stretching in the machine direction (MDX): under 2.0 MDX, the film usually suffers from uneven stretching mark defects and over 7.0 MDX, processing stability can be difficult to maintain, as the film may be prone to breakage at this high orientation rate.
Although crystalline propylene homopolymer and blends of crystalline propylene homopolymer with impact ethylene-propylene copolymers will display acceptable linear tear properties when oriented sufficiently in one direction, the higher content comonomer polypropylenes generally do not show acceptable linear tear properties by themselves. The reason for the use of copolymer and terpolymers is to get acceptably low seal initiation temperatures. Low seal initiation temperatures allow for faster sealing speeds and use of certain lamination films that may melt or deform if too high a sealing temperature is used. Sealing films made with crystalline propylene homopolymers as a component of the film, however, often show a higher-than-desired SIT and adjustments to raise sealer jaw temperature settings and/or to lower packaging machine linespeeds to accommodate such homopolymer-containing sealing films are often needed. Although such homopolymer-containing films can demonstrate excellent directional or linear tear properties with an amount of mono-orientation, SIT is often higher than desired. Although propylene copolymers can demonstrate excellent SIT, they often fail to exhibit satisfactory directional or linear tear properties when mono-oriented.
U.S. application Ser. No. 12/542,385 describes a linear or directional tear retortable sealant film using blends of metallocene-catalyzed propylene-butene elastomers and ethylene-propylene impact copolymers which are monoaxially oriented at least 4 times in the machine direction. Typical seal initiation temperatures reported are about 320° F. (160° C.) or higher. This reference is incorporated herein in its entirety.
U.S. patent RE30,726 describes a film including blended low density polyethylene and ionomer resins which is blow-extruded to form a film with linear tear properties in the direction of the extrusion.
U.S. Pat. No. 4,781,294 describes a tear-oriented package with one wall formed from foamed polypropylene and another from tear resistant polyester film or another such substance. However, zones of weakening, such as perforations, are used to provide easy opening of the packaging.
U.S. Pat. No. 6,248,442 describes the use of a multilayered film that includes a layer of LLDPE which is biaxially oriented through the process of machine direction orientation. This produces a bag which can be torn unidirectionally and which contains a resealable zipper.
U.S. Pat. No. 6,601,370 describes a process for forming a reclosable film package with a straight tear by attaching a nylon or polyester layer to a sealant layer, such as polyethylene. This involves two layers of overlapping films which propagate a tear along a linear path when sufficient force is applied.
U.S. Pat. No. 6,939,919 describes a blend of polypropylene and polyethylene with enhanced properties, of which a majority is polyethylene with a minority of the blend being polypropylene. However, this patent does not cite any linear tear properties of the resultant blend.
U.S. Pat. No. 6,541,086 describes a retort package design using an oriented polymer outer film (suitable for printing), an aluminum foil as a barrier film, a second oriented intermediate polymeric film, and a non-oriented polyolefin for the sealant film. Easy-tear functionality is added by surface-roughening the two oriented polymer films and overlapping them in a particular formation. The particular specific order of laminating the films and the surface-roughening by sandpaper provides for easy-tear properties and presumably directional tear, but this process involves additional films and extra steps to accomplish the desired tear properties.
U.S. Pat. No. 6,719,678 B1 describes a retort package design using multiple film layers whereby the intermediate layers (“burst resistant layer”) are scored by a laser such that the score lines provide an easy-tear feature and a directional tear feature.
U.S. Pat. No. 4,903,841 describes a retort package design that utilizes non-oriented cast polypropylene films as the sealable layer, which is surface-roughened or scored in a particular manner so as to impart directional tear properties.
U.S. Pat. No. 4,291,085 describes a retort package design using a non-drawn, non-oriented cast crystalline polypropylene film as the sealable layer with specific crystalline structure and orientation of the crystalline structures which must be less than 3.0. There are no directional tear properties cited.
U.S. Pat. No. 5,786,050 describes an “easy opening” pouch design which has as the inner ply (which contacts the pouch's contents) a sealant film including linear low density polyethylene; an intermediate layer composed of an oriented polyolefin with an MD/TD ratio of greater than 2; and an outermost layer of biaxially oriented PET or nylon film. The inner ply sealant of linear low density polyethylene is non-oriented. The specific orientation ratios of the intermediate film imparts easy-tear properties.
U.S. Pat. No. 4,834,245 describes a pouch design having a “tearing zone” using a monoaxially oriented film with a pair of notches aligned with the tearing direction and the direction of orientation of the film. The monoaxially oriented film that imparts the “tearing zone” is on the outside of the pouch and does not contact the pouch contents and is not designed or considered to be appropriate for heat-sealability.
U.S. patent application Ser. No. 11/596,776 describes a pouch design including at least one uni-directionally stretched film. The preferred embodiments describe a uni-directionally stretched polypropylene film or uni-directionally stretched polyethylene terephthalate film which imparts the easy-tear property. The application is silent as to the sealing properties of these layers or even which layer should be the sealant film.